1. Technical Field
The invention relates to a half baring, comprising a backing member and a least one metallic overlay, which is applied by means of electron beam vapor deposition and which comprises at least one finely dispersed component in a matrix material, the atomic weight of which component is greater than that of the matrix material. The invention also relates to a method of producing a half bearing such an overlay of a metallic alloy.
2. Related Prior Art
In general, sliding elements used for such purposes comprise multilayer composite systems of the following construction: steel support member serving as backing material, bearing metal layer of a Cu, Al or white metal alloy and a so-called sliding or third layer or overlay, which may be applied either by an electroplating process (E. Romer: Three-component bearings of GLYCO 40; GLYCO Engineering Report 8/67) or by a cathodic sputtering process as described in EP 0 256 226 B1. Layers applied by electroplating, which are generally based on Pb or Sn, exhibit the disadvantages of frequently inadequate corrosion resistance and low wear resistance. Furthermore, the electroplating process is in itself dubious from the environmental point of the view.
Where overlays are applied by the sputtering method, a considerable cost factor is introduced with respect to the complete sliding element, owing to the low deposition rates achievable therewith and the high technical complexity of the equipment needed.
GB 2270 927 describes aluminum alloys in which the SN content of the entire layer may be constant and lie between 10 and 80%. From Table 1 on pages 10 and 11 of this application it may be seen that, as the tin content increases, the possible limit load before the bearing demonstrates a tendency to corrode increases, while, on the other hand, the load-carrying capacity drops again drastically from a certain tin content. This specification does not contain any suggestions for improving running-in behavior. Sputtering is mentioned in this application as a production process for applying the overlay.
EP 0 376 368 B1 describes a very complex process for producing a bearing which is distinguished by good emergency running and running-in properties. This application also relates to aluminum-tin alloys, which are applied by means of a sputtering process. The nub of this application is that the particles incorporated in the metallic base material of the bearing alloy obey the rules of standard random distribution with respect to the diameter thereof, and that up to 1.0 wt. % oxygen is incorporated in the overlay, the micro-hardness of the overlay diminishing after heat treatment. In this way, the embeddability, emergency running properties and insensitivity to corrosion are improved.
WO 91/00375 describes a bearing whose overlay consists of a base material (e.g. aluminum) with a second phase (e.g. tin) dispersed finely therein. Here too a sputtering process is used. The aim of this invention is to produce a bearing whose overlay structure is such that the content of the second phase (e.g. tin) in the overlay increases in accordance with the thickness of the overlay continuously from 0% in the bottom layers to 100% in the top layers. This is effected on the one hand by the use of several targets of differing compositions or of varying sputtering parameters during coating. Overlays produced in this way exhibit very good properties from the point of view of their wear and fatigue behavior, which is achieved, however, by using a very complex process.
It is known from DE 195 14 835 A1 and 195 14 836 A1 to deposit overlays on concavely curved sliding elements by means of electron beam vapor deposition, wherein in both specifications the formation of particular layer thickness profiles is a priority. In order to achieve a uniform layer thickness for half bearings, according to DE 195 14 835 A1 the evaporating apparatus and backing member are moved relative to one another in linear manner and at different speeds during evaporation coating. To this end, appropriate adjusting means are required inside the coating chamber. The intention of DE 195 14 836 A1, on the other hand, is to produce a non-uniform layer thickness. The layer thickness of the sliding element is at its greatest in the apex area and reduces continuously towards the partial surfaces. In order to achieve this, the method provides that a distance be set between the evaporating apparatus and the apex area of the half bearing of 150 to 350 mm, that during evaporation coating of the layer the evaporating apparatus the backing member be positioned in fixed relation to each other and that the condensation rate for deposition in the apex area be set to at least 80 nm/s.
A method is known from DE 36 06 529 A1 for producing multilayer materials or multilayer workpieces by the vapor deposition of at least one metallic material onto a metallic substrate, an electron beam vapor deposition process likewise being used to apply the overlay. The method is carried out in a residual gas atmosphere under pressures ranging from 10.sup.-2 -10.sup.-3 mbar, wherein the material is dispersion-hardened or dispersion-strengthened simultaneously with the vapor deposition. Coating rates are set at approximately 0.3 .mu.m/s. During vapor deposition, the substrate is kept at a temperature between 200.degree. C. and 800.degree. C. The temperature of the substrate is 200.degree. C. to 300.degree. C. for vapor deposition of aluminum alloys and in the range of from 500.degree. C. to 700.degree. C. for vapor deposition of copper-lead alloys. The load-carrying capacity of the layers produced according to this method is markedly better than that of layers produced by powder-metallurgical methods. A priority of this application is to produce a defined hard phase content in the overlay by dispersion strengthening, e.g. by producing oxides during vapor deposition.
These three specifications relating to electron beam vapor deposition do not give any indications as to how to achieve different distributions of the alloy components. The load-carrying capacity or running-in behavior are not adequate for some applications.